Add slots of channel and Add get_dev_pts_max_id()  for hotplug serial test 

1.libvirt_xml.devices: Add slots for channel.
    The latest slots includes 'alias' and 'address'.

2.  virttest.utils_misc: add function to get the maxi ID for pty chardev
        when hot-pluging in pseudoterminal interfaces to VM for pty device,
    we must specify the device source path (/dev/pts/<number>) for it.
    The function get_dev_pts_max_id() can used to get the maxi and
    occupied device number.
    and then, user can specify the available source path based on it.



Maybe somebody doubt the definition and format for new slots "alias" and 'address'  and they aren't mentioned by libvirt.org.
so i list the output about it, hope some assist  for review.
[root@localhost virt-test]# ./run -t libvirt --no-downloads -k --keep-image-between-tests  --tests "type_specific.io-github-autotest-libvirt.hotplug_serial.at_dt_device.pty_dev.normal_test" -v
<-----------------------------cut------------------------->
17:16:55 DEBUG| Channel XML:
<?xml version='1.0' encoding='UTF-8'?>
<channel type="pty"><source path="/dev/pts/10" /><target name="pty" type="virtio" /><address bus="0" controller="0" type="virtio-serial" /><alias name="pty" /></channel>
<-----------------------------cut------------------------->
17:16:57 INFO | PASS type_specific.io-github-autotest-libvirt.hotplug_serial.at_dt_device.pty_dev.normal_test
17:16:57 INFO | 
 PASS (14.61 s)